[3.16.y-ckt stable] Patch "serial: 8250_dw: Fix deadlock in LCR workaround" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    serial: 8250_dw: Fix deadlock in LCR workaround

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt10.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From 59f938497062e849e6ada99b6d9b1dc6c9d7d6b1 Mon Sep 17 00:00:00 2001
From: Peter Hurley <peter at hurleysoftware.com>
Date: Wed, 11 Mar 2015 09:19:16 -0400
Subject: serial: 8250_dw: Fix deadlock in LCR workaround

commit 7fd6f640f2dd17dac6ddd6702c378cb0bb9cfa11 upstream.

Trying to write console output from within the serial console driver
while the port->lock is held causes recursive deadlock:

  CPU 0
spin_lock_irqsave(&port->lock)
printk()
  console_unlock()
    call_console_drivers()
      serial8250_console_write()
        spin_lock_irqsave(&port->lock)
** DEADLOCK **

The 8250_dw i/o accessors try to write a console error message if the
LCR workaround was unsuccessful. When the port->lock is already held
(eg., when called from serial8250_set_termios()), this deadlocks.

Make the error message a FIXME until a general solution is devised.

Cc: Tim Kryger <tim.kryger at gmail.com>
Reported-by: Zhang Zhen <zhenzhang.zhang at huawei.com>
Signed-off-by: Peter Hurley <peter at hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh at linuxfoundation.org>
[ luis: backported to 3.16:
  - dropped changes to dw8250_serial_outq() ]
Signed-off-by: Luis Henriques <luis.henriques at canonical.com>
---
 drivers/tty/serial/8250/8250_dw.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_dw.c b/drivers/tty/serial/8250/8250_dw.c
index 51b307aab75e..f4e14f9baae5 100644
--- a/drivers/tty/serial/8250/8250_dw.c
+++ b/drivers/tty/serial/8250/8250_dw.c
@@ -165,7 +165,10 @@ static void dw8250_serial_out(struct uart_port *p, int offset, int value)
 			dw8250_force_idle(p);
 			writeb(value, p->membase + (UART_LCR << p->regshift));
 		}
-		dev_err(p->dev, "Couldn't set LCR to %d\n", value);
+		/*
+		 * FIXME: this deadlocks if port->lock is already held
+		 * dev_err(p->dev, "Couldn't set LCR to %d\n", value);
+		 */
 	}
 }

@@ -202,7 +205,10 @@ static void dw8250_serial_out32(struct uart_port *p, int offset, int value)
 			dw8250_force_idle(p);
 			writel(value, p->membase + (UART_LCR << p->regshift));
 		}
-		dev_err(p->dev, "Couldn't set LCR to %d\n", value);
+		/*
+		 * FIXME: this deadlocks if port->lock is already held
+		 * dev_err(p->dev, "Couldn't set LCR to %d\n", value);
+		 */
 	}
 }